Opus numbers are purely a means to identify works by some composers, no more & no less. Beethoven's piano sonatas are no longer identified by "No.xy
" anymore, because new editions now have 34 'sonatas' (including two early sonatas) rather than the 32 that used to be commonly numbered; and in any case, the Op.49 sonatas are very early works and Luddy never wanted them to be part of his sonata cycle. So, do you deduct the Op.49 and include the other two to get back to 32, or include all
of them, like in the new Associated Board edition? Or adhere to the composer's wishes, and only assign the numbers to the pieces he himself deemed to be proper piano sonatas - i.e. 30 of them?
Composers often assign opus numbers starting with pieces they deemed 'mature', or 'good' enough to be given such numbers (thereby leaving a good number of other pieces in limbo - like Beethoven's works without opus numbers (WoO) - e.g. the very famous WoO 59
), or their publishers may assign opus numbers based on the order of publication. Which is why Chopin's piano concertos (among others) got "misnumbered" (his Op.21 is earlier than his Op.11).
As for Mozart, Köchel's numbering system has come into widespread use, trying to include everything Wolfie ever wrote - because he wrote a lot of stuff as a child (& also copied a lot of other people's stuff), plus other pieces he just dashed off and wouldn't have deigned them with opus numbers (if he'd used them). But his piano sonatas are
numbered, just that they aren't often used (like No.16 in C, K545).
Similarly, Kirkpatrick's numbering for Scarlatti has largely replaced Longo; and Deutsche's for Schubert has largely replaced the opus numbers (which only had the published works).
BTW, many contemporary composers don't bother with opus numbers for their works - they just give them unique names, like Laterna Magica
, or Mavis in Vegas